Optical spectroscopy may be used to detect and quantify characteristics of a source, such as the average spectrum of the source or the density of chemical and biological species of the source. As used herein, the term “source” refers to the object being spectrally analyzed. In determining chemical composition of a source, the spectral signature of the target species may be encoded on the optical field by mechanisms including absorption, admission, inelastic scattering, fluorescence, and wave mixing. Many sources are referred to as large etendue because they reflect or scatter light in a large solid angle over a large area. The etendue of a source is a measure of both the spatial extent of the source and the solid angle in to which it radiates. Large etendue sources are also referred to as incoherent or diffuse sources because low spatial coherence is implicit in a large solid angle radiated over a large area. An example of a large etendue source is laser-illuminated biological tissue.
One problem with measuring light radiated from large etendue sources using conventional spectrometers is that conventional spectrometers use filters that decrease optical throughput. For example, a conventional multiplex spectrometer measures light emanating from a source using elemental detectors with a narrowband color filter placed over each detector. Using narrowband color filters over each detector reduces the optical throughput of the conventional spectrometer. As a result, conventional multiplex spectrometers that utilize narrowband filters are incapable of accurately determining the optical properties of diffuse sources.
In another type of conventional spectrometer, multimodal measurements are taken in series and the measurements are combined to estimate the optical properties of a diffuse source. The spectrometers that perform such measurements are referred to as scanning spectrometers. Using scanning spectrometers to measure diffuse sources is disadvantages because such detectors require microelectromechanical and/or piezoelectric components in order to successively apply different spectral filters to the detectors, and such components are expensive and difficult to fabricate.
A further disadvantage of scanning spectrometers is that taking multiple measurements in series increases measurement time. Increasing measurement time may be undesirable for some types of measurements, such as in vivo tissue measurements.
Yet another disadvantage of scanning spectrometers is lack of intelligent spectral filters. Conventional scanning spectrometers typically capture the full spectrum of electromagnetic radiation. Capturing the full spectrum is inefficient because some measurements contain radiation bands that are not of interest to analyzing a particular source.
Accordingly, in light of the difficulties associated with conventional spectroscopy, there exists a need for improved methods and systems for multimode multiplex spectroscopy capable of accurately and efficiently measuring characteristics of large etendue sources.